1. Field of the Invention
This invention is related to a motor-starter and more particularly to a combination motor-starter circuit breaker wherein a single set of contacts are used for operation and protection of the motor and its associated circuit.
2. Description of the Prior Art
Prior art combination motor-starter as exemplified by U.S. Pat. No. 3,638,157 issued to Z. J. Kruzic and assigned to the assignee of the present application are provided with separate units which provide for (1) motor-starting and stopping; (2) current interruption; and, (3) current limiting.
It is old in the art to provide a plurality of motor control and circuit protection devices in series in a common enclosure for motor circuit protection. It is also old in the art to provide a fused switch motor starter combination comprising: a contactor with overload relays for automatically opening the contactor and the motor circuit upon the occasion of lesser overload currents caused by motor overload, current limiting fuses for interrupting low level to high level fault current, and a manually operable switch for permitting manual opening and closing of the motor circuit.
It is also old to provide a fused breaker motor starter combination comprising: a contactor with an overload relay for opening the motor circuit upon the occurrence of low overload currents; a manually operable circuit breaker automatically operable to interrupt the motor circuit upon the occurrence of low level to high level faults; and current limiting fuses that operate upon the occurrence of a heavy short circuit current. Prior art motor-starters normally are a dual break device which usually have silver cadmium oxide contacts. A solenoid mechanism is used to close the contacts. Contacts are spring biased open and are automatically reopened if the system voltage is lost. Low to high level fault protection is provided by a series connected molded case circuit breaker. The circuit breaker can be a single break Deion circuit breaker with an adjustable magnet trip. For long life, durability, and erosion and weld resistance, silver tungsten contacts are often used in the circuit breaker. The adjustable magnetic trip is set to operate the breaker at from 7 to 13 times full load current. The molded case circuit breaker is capable of operation to interrupt high overload currents. For example, in a NEMA Size One device having a 100 ampere rating, the circuit breaker can interrupt currents up to 15,000 amperes up to three times. Current limitation is provided by current limiting fuses connected in series with the contactor and molded case circuit breaker for protection against possible faults up to 100,000 amperes RMS symmetrical. In addition a thermal trip relay which can be adjusted to open the motor-starter at currents below the molded case circuit breaker trip level with an appropriate delay are often provided. It is also common to provide a series connected disconnect switch.
It can be seen that to obtain full motor circuit protection in the prior art a plurality of series connected devices are necessary. This prior art construction is expensive and the power loss across the plurality of devices is greater than that which occurs across a single motor contactor. Also, it is necessary to change fuses after a high current limiting operation.